1. Field of the Invention
The present invention relates to a device having a light-transmitting electrode and a pad electrode which are formed on a p-type GaN related compound semiconductor layer.
2. Description of the Related Art
In conventional compound semiconductors, an ohmic contact is obtained by depositing metals on the semiconductor surface and heating the metals to convert the same to an alloy and to cause metal diffusion into the semiconductor, because an ohmic contact is not obtainable by the mere deposition of metals.
Even when p-type GaN related compound semiconductors are subjected to a treatment for reducing resistance, e.g., irradiation with electron beams, the thus-treated semiconductors still have higher resistivities than n-type GaN related compound semiconductors. Consequently, in such p-type GaN related compound semiconductors, the p-type layer has almost no current flow in lateral directions, and only the part thereof directly beneath the electrode emitts light.
Under these circumstances, a current-diffusing electrode having light transmission properties and ohmic properties has been proposed which is formed by depositing a nickel (Ni) layer and a gold (Au) layer, each having a thickness of several tens of angstroms, (xc3x85) and heating the metal layers (see Japanese Unexamined Patent Publication No. Hei. 6-314822).
However, the electrode formed by depositing nickel (Ni) and gold (Au) each having a thickness of several tens of angstroms and heating the metals poses a problem that the light-emitting pattern quality deteriorates with the lapse of time, resulting in an increased driving voltage. However, the electrode has satisfactory optical and electrical characteristics in the initial stage.
The reason for the quality deterioration is believed to be as follows. Since the nickel (Ni) and gold (Au) deposited layers are extremely thin, part of the nickel (Ni) is replaced by gold (Au) during the heat treatment, and the nickel (Ni) exposed on the electrode surface oxidizes to form NiO. When current is caused to flow through the electrode in this state, the NiO reacts with an OHxe2x88x92 group of water present in the surrounding atmosphere to form a substance comprising NiOOH, as shown by the following scheme (1). Since NiOOH has poor wettability by gold (Au) and by the GaN related compound semiconductor, the NiOOH aggregates. As a result, light-emitting pattern quality deteriorates with the lapse of time and the contact resistance of the electrode increases. Thus, conventional art devices employing the proposed electrode are believed to deteriorate in optical and electrical characteristics.
NiO+OHxe2x88x92xe2x88x92NiOOH+exe2x88x92xe2x80x83xe2x80x83(1)
Further, since this current-diffusing electrode is thin, a pad electrode made of Ni/Au or Au is formed thereon for bonding.
However, the conventional art device described above has insufficient adhesion between the pad electrode and the current-diffusing electrode. Hence, if the surface of the current-diffusing electrode on which a pad electrode is to be formed has been soiled, there is a problem that the finally obtained device has problems such as the peeling of the pad electrode and a poor light-emitting pattern. In addition, even if the pad electrode has satisfactory adhesion to the current-diffusing electrode, the light emission occurring in the shade of the bonding pad cannot be directly observed, unavoidably resulting in a light emission loss.
Further, there is still another problem as follows.
In conventional GaN related compound semiconductors, low-resistivity p-type conduction is not obtainable by mere doping with a p-type impurity. It has hence been proposed to impart p-type low resistance to a GaN related compound semiconductor doped with a p-type impurity by irradiating the doped semiconductor with electron beams (see Japanese Unexamined Patent Publication No. Hei. 2-257679) or by subjecting the doped semiconductor to thermal annealing (see Japanese Unexamined Patent Publication No. Hei. 5-183189). It has also been proposed to conduct the thermal annealing for imparting p-type low resistance simultaneously with alloying for forming an electrode (see Japanese Unexamined Patent Publication No. Hei. 8-51235).
However, in the method using thermal annealing described in Japanese Unexamined Patent Publication No. Hei. 5-183189, the heat treatment should be conducted at a temperature not lower than 700xc2x0 C. in order to obtain a saturated low resistivity. Although this kind of semiconductor has conventionally employed aluminum as the main electrode material, use of a temperature not lower than 700xc2x0 C. for electrode alloying produces problems, such as the formation of aluminum balls resulting from aluminum melting, an impaired surface state, increased contact resistance of the electrode, and wire bonding failure.
Consequently, the heat treatment for electrode alloying should be conducted at a relatively low temperature of from 500 to 600xc2x0 C. It is, however, noted that the heat treatment for imparting p-type low resistance does not result in a sufficiently low resistivity when conducted at a temperature in the range of from 500 to 600xc2x0 C. It has hence been necessary to conduct the heat treatment for imparting p-type low resistance and the heat treatment for electrode alloying as separate steps, respectively.
On the other hand, Japanese Patent Publication No. Hei. 8-51235 proposes to conduct the impartation of p-type low resistance simultaneously with electrode alloying by performing a heat treatment at a temperature of from 400 to 800xc2x0 C. However, this method has the following problems. The impartation of p-type low resistance is insufficient in the low-temperature range where electrode alloying is achieved satisfactorily. In the high-temperature region suitable for the sufficient impartation of p-type low resistance, electrode alloying cannot be conducted satisfactorily, resulting in increased contact resistance and poor ohmic properties.
In view of the problems described above, an object of the present invention is to realize a GaN related compound semiconductor light-emitting device which has light transmission properties and ohmic properties and retains a stable light-emitting pattern and a constant driving voltage over a long period of time, and to realize processes for producing the device.
Another object of the present invention is to impart p-type low resistance to a GaN related compound semiconductor through a heat treatment so that a saturated low resistivity value can be realized using a lower temperature for the treatment.
Still another object of the present invention is to realize the impartation of p-type low resistance at a lower temperature to thereby sufficiently impart p-type low resistance and obtain an electrode having low contact resistance and satisfactory ohmic properties, even when the heat treatment for imparting p-type low resistance and that for electrode alloying are conducted as the same step.
Still another object of the present invention is to improve the adhesion between a pad electrode and a current-diffusing electrode to thereby prevent the pad electrode from peeling off and, at the same time, to form a high-resistivity region under the pad so that current flows in the current-diffusing electrode selectively through areas other than that under the pad to thereby diminish light emission under the pad and attain effective utilization of light emission.
The above-described problem is eliminated with the light-emitting device of the present invention according to a first aspect of the present invention. This light-emitting device has a p-type GaN related compound semiconductor layer having formed thereon an electrode which transmits light to the semiconductor layer and which is a metal layer comprising a cobalt (Co) alloy, palladium (Pd), or a palladium (Pd) alloy. Since the elements constituting the electrode are unsusceptible to oxidation, not only is the electrode prevented from suffering the light-emitting pattern change with time caused by electrode oxidation to thereby give a stable light-emitting pattern over a long period of time, but also the electrode can have reduced contact resistance to thereby enable a constant driving voltage over a long period of time. In addition, since cobalt (Co) and palladium (Pd) each is an element having a large work function, satisfactory ohmic properties are obtained.
The metal layer comprising a cobalt (Co) alloy may be formed from one member selected from the group consisting of a two-layer structure comprising a first metal layer made of cobalt (Co) and a second metal layer made of gold (Au) formed on the first metal layer, a two-layer structure comprising a first metal layer made of gold (Au) and a second metal layer made of cobalt (Co) formed on the first metal layer, and an alloy layer made of cobalt (Co) and gold (Au), by alloying the one member through a heat treatment. This metal layer is free from the problem in electrodes made of cobalt (Co) alone that the light-emitting pattern changes with the lapse of time because of the susceptibility of cobalt (Co) to oxidation. Specifically, the electrode formed by heating a two-layer structure comprising a layer made of cobalt (Co) and a layer made of gold (Au) or by heating a layer of an alloy of cobalt (Co) with gold (Au) is prevented from undergoing cobalt (Co) oxidation, has reduced contact resistance, enables a stable light-emitting pattern over a long period of time, and has excellent light transmission properties.
An electrode which has reduced contact resistance, enables a stable light-emitting pattern over a long period of time, and has excellent light transmission properties is also obtained from a three-layer structure comprising a first metal layer made of cobalt (Co), a second metal layer made of a group II element formed on the first metal layer, and a third metal layer made of gold (Au) formed on the second metal layer, by alloying the three-layer structure through a heat treatment, or obtained from a two-layer structure comprising a first metal layer made of cobalt (Co) and a second metal layer made of an alloy of palladium (Pd) with platinum (Pt) formed on the first metal layer, by alloying the two-layer structure through a heat treatment. Effective examples of the group II element include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), zinc (Zn), and cadmium (Cd).
The metal layer comprising a palladium (Pd) alloy may be formed from either a two-layer structure comprising a first metal layer made of palladium (Pd) and a second metal layer made of gold (Au) formed on the first metal layer, or a two-layer structure comprising a first metal layer madeiof gold (Au) and a second metal layer made of palladium (Pd) formed on the first metal layer, by alloying the two-layer structure through a heat treatment. Thus, an electrode is obtained which has reduced contact resistance, enables a stable light-emitting pattern over a long period of time, and has excellent light transmission properties.
An electrode which has reduced contact resistance, enables a stable light-emitting pattern over a long period of time, and has excellent light transmission properties is obtained also from a layer made of an alloy of palladium (Pd) with platinum (Pt) by alloying the layer through a heat treatment.
A metal layer may be formed on a p-type GaN related compound semiconductor layer through a heat treatment conducted at a temperature from 400 to 700xc2x0 C. The metal layer formed can be a satisfactorily alloyed layer. Thus, an electrode having stable light-emitting properties and stable electrical characteristics can be obtained.
A metal layer having reduced contact resistance can be formed through a heat treatment conducted under low-vacuum conditions. The term xe2x80x9clow-vacuum conditionsxe2x80x9d used herein means a pressure of 10 Torr or lower.
A metal layer having reduced contact resistance can be formed through a heat treatment without reducing light-emitting pattern quality, by conducting the heat treatment in an atmosphere comprising at least oxygen (O2) or a gas containing oxygen (O), or by conducting the heat treatment in an inert gas atmosphere. The term xe2x80x9catmosphere comprising oxygen (O2)xe2x80x9d as used herein include 100% oxygen (O2). The term xe2x80x9cgas containing oxygen (O)xe2x80x9d means CO, CO2, etc. Effective examples of the inert gas contemplated by the present invention include nitrogen (N2), helium (He), neon (Ne), argon (Ar), and krypton (Kr).
Further, the above-described problem is eliminated with the process for producing a p-type GaN related compound semiconductor of the present invention according to a second aspect of the present invention. This process for producing a p-type GaN related compound semiconductor comprises subjecting a GaN related compound semiconductor doped with a p-type impurity to a heat treatment in a gas comprising at least oxygen.
Further, the above-described problem is eliminated by the process for producing a p-type GaN related compound semiconductor having a p-type GaN related compound semiconductor layer and an electrode according to a third aspect of the present invention. This process for producing a GaN related compound semiconductor device having a p-type GaN related compound semiconductor layer and an electrode comprises: forming a layer of a GaN related compound semiconductor doped with a p-type impurity; forming an electrode on the GaN related compound semiconductor layer; and subjecting the GaN related compound semiconductor layer having the electrode formed thereon to a heat treatment in a gas comprising at least oxygen.
Furthermore, the above-described problem is eliminated by the process for producing GaN related compound semiconductor having a p-type GaN related compound semiconductor layer, an n-type GaN related compound semiconductor layer, and two electrodes respectively for these layers according to a fourth aspect of the present invention. This process for producing a GaN related compound semiconductor device having a p-type GaN related compound semiconductor layer, an n-type GaN related compound semiconductor layer, and two electrodes respectively for these layers comprises:
forming a first electrode on the GaN related compound semiconductor layer doped with a p-type impurity, and forming a second electrode on the n-type GaN related compound semiconductor; and subjecting the resultant structure to a heat treatment in a gas comprising at least oxygen.
The term xe2x80x9cGaN related compound semiconductorxe2x80x9d means a compound which is based on GaN and contains one or more group III elements, e.g., In and Al, by which part of the gallium has been replaced. An example of the GaN related compound semiconductor is a four-element compound represented by the general formula (AlxGa1xe2x88x92x)yIn1xe2x88x92yN (Oxe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61).
The gas comprising oxygen used in each of the processes according to the present invention may be at least one member selected from O2, O3, CO, CO2, NO, N2O, NO2, and H2O or a mixed gas comprising two or more of these members. The gas comprising oxygen may also be a mixed gas comprising at least one of O2, O3, CO, CO2, NO, N2O, NO2, and H2O and one or more inert gases, or be a mixed gas comprising a mixture of two or more of O2, O3, CO, CO2, NO, N2O, NO2, and H2O and one or more inert gases. In short, the gas comprising oxygen means a gas containing oxygen atoms or a gas of molecules containing oxygen atoms.
The pressure of the atmosphere in which the heat treatment is conducted is not particularly limited as long as the GaN related compound semiconductor is not pyrolyzed at the temperature used for the heat treatment. In the case where O2 gas alone is used as the gas comprising oxygen, the gas may be introduced at a pressure higher than the decomposition pressure for the GaN related compound semiconductor. In the case where a mixture of O2 with an inert gas is used, the pressure of the whole mixed gas is regulated to a value higher than the decomposition pressure for the GaN related compound semiconductor. In this case, an O2 gas proportion not smaller than about 10xe2x88x926 based on the whole mixed gas is sufficient. In short, an extremely small amount of oxygen suffices to the gas comprising oxygen for the reason which will be given later. There is no particular upper limit on the amount of the gas comprising oxygen introduced from the standpoints of the impartation of p-type low resistance and electrode alloying. Any high pressure is usable as long as production is possible.
The most preferred range of the temperature for the heat treatment is from 500 to 600xc2x0 C. As will be described later, a p-type GaN related compound semiconductor having a completely saturated resistivity can be obtained at temperatures not lower than 500xc2x0 C. At temperatures not higher than 600xc2x0 C., the alloying treatment of an electrode can be conducted satisfactorily.
Preferred temperature ranges are from 450 to 650xc2x0 C., from 400 to 600xc2x0 C., and from 400 to 700xc2x0 C. The lower the temperature, the higher the p-type resistivity. The higher the temperature, the poorer the electrode properties and the higher the possibility for thermal deterioration of crystals.
The first electrode desirably comprises a metal layer which comprises a cobalt (Co) alloy, palladium (Pd), or a palladium (Pd) alloy and has light transmission properties and ohmic properties. This metal layer comprising a cobalt (Co) alloy is a layer formed from a two-layer structure comprising a first metal layer made of cobalt (Co) and a second metal layer made of gold (Au) formed on the first metal layer, from a two-layer structure comprising a first metal layer made of gold (Au) and a second metal layer made of cobalt (Co) formed on the first metal layer, or from a layer of an alloy of cobalt (Co) with gold (Au), by alloying the same through a heat treatment. Alternatively, the metal layer comprising a cobalt (Co) alloy is a layer formed from a three-layer structure comprising a first metal layer made of cobalt (Co), a second metal layer made of a group II element formed on the first metal layer, and a third metal layer made of gold (Au) formed on the second metal layer, by alloying the three-layer structure through a heat treatment. The metal layer comprising a palladium (Pd) alloy is a layer formed from a two-layer structure comprising a first metal layer made of palladium (Pd) and a second metal layer made of gold (Au) formed on the first metal layer or from a two-layer structure comprising a first metal layer made of gold (Au) and a second metal layer made of palladium (Pd) formed on the first metal layer, by alloying the two-layer structure through a heat treatment.
The first electrode can be a layer formed by alloying, through a heat treatment, a two-layer structure comprising a first metal layer made of nickel (Ni) and a second metal layer made of gold (Au) formed thereon.
The above-described materials of the first electrode have been selected so as to result in satisfactory properties with respect to contact resistance with p-type GaN related compound semiconductors, light-emitting pattern, property change with time, junction strength, and ohmic properties.
The second electrode desirably comprises aluminum (Al) or an aluminum alloy. These electrode materials have been selected from the standpoints of contact resistance with n-type GaN related compound semiconductors and ohmic properties.
In the process according to the second aspect of the present invention, a gas comprising oxygen is used as the surrounding gas for the heat treatment. As a result, it has become possible to use a lower temperature for obtaining a GaN related compound semiconductor having p-type low resistance. As will be described later, use of temperatures not lower than 500xc2x0 C. resulted in a saturated low value of resistivity. The resistivity began to decrease at around 400xc2x0 C. At 450xc2x0 C., the resistivity was about a half of that at 400xc2x0 C.
In the processes according to the third and fourth aspects of the present invention, a saturated low resistivity suitable for practical use is obtained at lower temperatures as described above. Consequently, the heat treatment for imparting p-type low resistance and the heat treatment for electrode alloying can be carried out as the same step. As a result, processes for device production can be simplified. In addition, since the heat treatment can be conducted at a low temperature, thermal deterioration of devices can be alleviated.
With respect to the fact that the heat treatment in a gas comprising oxygen is effective in imparting low resistance at lower temperatures, the following explanation is given by the present inventors. A GaN related compound semiconductor cannot be made to have p-type low resistance by merely doping the same with a p-type impurity, e.g., magnesium. This is because the atoms of the p-type impurity are bonded to hydrogen atoms and, hence, do not function as an acceptor. It is therefore thought that upon the removal of the hydrogen atoms bonded to the atoms of the p-type impurity, the impurity comes to function as an acceptor. When a heat treatment is conducted in a gas comprising oxygen, the separation of the impurity atoms from the hydrogen atoms is thought to be catalyzed by the oxygen. As a result, semiconductor devices having a reduced resistivity are obtainable at lower temperatures.
Still further, the above-described problem is eliminated with the GaN related compound semiconductor device having a p-type GaN related compound semiconductor according to a fifth aspect of the present invention. This GaN related compound semiconductor device having a p-type GaN related compound semiconductor comprises: a current-diffusing electrode having light transmission properties which has been formed on the p-type GaN related compound semiconductor and a pad electrode for bonding which has been formed on the current-diffusing electrode and contains at least one metal reactive with nitrogen. The device further includes a high-resistivity region on the p-type GaN related compound semiconductor in its part located under the pad electrode, the high-resistivity region having been formed through an alloying treatment by the reaction of the metal with the p-type GaN related compound semiconductor.
According to this device, a current-diffusing electrode having light transmission properties is formed on a p-type GaN related compound semiconductor, and a pad electrode containing at least one metal reactive with nitrogen is formed thereon.
In an alloying treatment, the metal reactive with nitrogen which is contained in the pad electrode reacts with the p-type GaN related compound semiconductor. As a result, the adhesion between the pad electrode and the current-diffusing electrode as well as between the pad electrode and p-type GaN surface is improved and the pad electrode can be prevented from peeling off. The reaction between the metal reactive with nitrogen contained in the pad electrode and the GaN related compound semiconductor further produces an effect that since the reaction generates nitrogen holes within part of the GaN related compound semiconductor, the donor attributable to these holes in that part compensates for an acceptor to thereby form a high-resistivity region in that part of the semiconductor. Consequently, current flows from the pad electrode not downward but in lateral directions along the current-diffusing electrode. Since the region of a pad electrode originally has a large thickness and no light transmission properties, it is virtually impossible to take light out of the device through the pad electrode or to cause external light to strike on the semiconductor through the pad electrode. According to the present invention, only the part where light is effectively utilizable can have an improved current density and, as a result, the effective efficiency of electricity-to-light conversion or light-to-electricity conversion is improved.